The biggest difference between the two verbs is that lay is a transitive verb, while lie is an intransitive verb. And you know the difference between a transitive and intransitive verb, right?
A transitive verb is one that takes a direct object, i.e., there is something that receives the action of the verb.
Example: Please lay the book on the table.
The verb lie is an intransitive verb, i.e., it cannot take a direct object. You cannot “lie” a book, or anything else, down.
The reason people have such a problem with lay and lie is due to the fact that the past tense of the verb “lie” is “lay,” which, of course, is the present tense of the verb “lay.”
Every verb has three principle parts – the infinitive, the simple past tense, and the past participle.
Below are the principle parts of the verbs lie and lay:
It’s correct to say, “I lay in bed all day,” but it’s incorrect to say, “I will lay in bed until my headache goes away.”
So, you need to lie down today, yesterday you lay down, in the past you have lain down.
Today, you lay the book on the table, yesterday you laid the book on the table, and in the past, you have laid the book on the table.
Now, I do hope I have laid out the differences between lie and lay clearly enough so the proper uses of both verbs lie in the back of your mind, so they’ll be available to you when you need them.